Thermal control in a method of bidirectionally attenuating glass in a float process

ABSTRACT

Optical quality of float glass is improved by attenuating longitudinally prior to attenuating laterally and by carrying out each attenuation step within specified temperature ranges.

RELATED APPLICATION

This application is a continuation-in-part of U.S. Patent ApplicationSer. No. 137,329 filed Apr. 4, 1980, now U.S. Pat. No. 4,305,745, thedisclosure of which is hereby incorporated by reference.

BACKGROUND OF THE INVENTION

This invention relates to a method for manufacturing flat glass whereinthe glass is formed into a flat sheet while supported on a surface of apool of molten metal, commonly referred to as the float process. Moreparticularly, this invention relates to a process for attenuating theglass while supported on the molten metal to a thickness below theequilibrium thickness of the glass in such a manner so as to minimizedistortion in the product glass.

In a float forming process, molten glass is delivered onto a pool ofmolten metal and thereafter formed into a continuous ribbon or sheet ofglass as disclosed, for example, in U.S. Pat. No. 710,357 of Heal; U.S.Pat. No. 789,911 of Hitchcock; U.S. Pat. Nos. 3,083,551 and 3,220,816 ofPilkington; and U.S. Pat. No. 3,843,346 of Edge et al. Under thecompeting forces of gravity and surface tension, the molten glass on themolten metal spreads outwardly to an equilibrium thickness of about 0.27inches. In order to produce glass of thicknesses less than theequilibrium thickness, the prior art has resorted to variousarrangements for stretching the glass while still in a viscous state onthe pool of molten metal. The simplest stretching technique is thatshown in U.S. Pat. No. 3,215,516 of Pilkington wherein stretching isdone in the longitudinal direction (the direction of glass travel) only,wherein the stretching force is provided by the tractive meanswithdrawing the glass from the float chamber. In such an arrangement,the ribbon loses width as it becomes thinner. A common refinement ofthis arrangement is to employ lateral stretching means in order toreduce the loss of ribbon width as it is being stretched longitudinally.Typical of this latter approach is the process shown in U.S. Pat. No.3,695,859 to Dickinson et al. Another approach is to maintain the ribbonof glass at essentially constant width by applying lateral tractiveforces to edge portions of the ribbon as the ribbon is being attenuatedin the longitudinal direction as exemplified in U.S. Pat. No. 3,843,346of Edge et al.

Process perturbations originating with the attenuating process affectthe topography of the glass ribbon in ways that degrade the opticalquality of the product glass. The topography of float glass ischaracterized by two types of elongated features, thickness variationsand corrugations, which extend generally parallel to the direction ofglass travel, i.e., the longitudinal direction. These deviations fromperfect flatness are, in effect, cylindrical lenses which distort lightreflected from and/or transmitted through the product glass sheet.Analysis of the distortion patterns using optical scanners in adirection transverse to the direction that the glass traveled in theforming process reveals that the distortion patterns can be consideredas consisting of randomly superimposed sinusoidal waves whosewavelengths vary over a wide range. It has also been found that thedominant component of the instrumentally measured signal correspondingto transmitted light occurs at rather well defined wavelengths that mayrange from about 1.2 to 1.4 inches (3.0 to 3.6 centimeters) for a"constant width" float forming process as in the Edge et al. patentcited above, to about 0.25 to 1.0 inches (0.6 to 2.5 centimeters) in thefreefall type of float forming as in the Pilkington patents cited above.Furthermore, these dominant wavelengths have been found to lie within arange to which the human eye is particularly sensitive for mostapplications.

Surface distortion in float glass is believed to arise from severalcategories of perturbations. First, inhomogeneities in the glasscomposition ("ream") not only cause nonuniformity of the refractiveindex of the glass but also can contribute to surface distortion.Second, thermal nonuniformity either in the molten glass entering thefloat forming chamber or within the chamber itself can contribute tosurface distortion. Third, variations in the flow of molten glass fromthe melter to the forming chamber, either volume flow rate fluctuationsor inequalities in the thickness of entering molten glass across thewidth of the ribbon of glass. Fourth, mechanical perturbations fromcontact of various members of the forming apparatus with the deformableglass ribbon. These include, for example, the stretching machines andside barriers as well as fluctuations in the speed with which thedimensionally stable ribbon is withdrawn from the forming chamber. Theseperturbations in the glass/tin system generate thickness variations orcorrugations in the glass through a variety of mechanisms such asdifferential stretching, viscous folding, wrinkling, embossing, andmembrane stress. While minimizing the causes of these perturbations isdesirable, such an approach is limited because the perturbations cannotbe completely eliminated, particularly in the case where less thanequilibrium thickness glass is being produced. Therefore, this inventionrelates to diminishing the effects on distortion of these perturbationsrather than eliminating the perturbations themselves.

As a newly formed ribbon of glass still in a softened conditionprogresses along the molten tin bath its topography is continuallychanging as perturbations introduce new defects into the ribbon andpreviously introduced defects are changed in shape. A defect maydecrease in amplitude by means of viscous decay, or the wavelength of adistortion pattern may be altered by extensive or compressive stresses.It would be desirable if these distortion decaying mechanisms could becoordinated with the attenuating process so as to minimize the amount ofdistortion imparted to the glass when attenuated to below equilibriumthicknesses.

Some attempts have been made in the prior art to correlate the manner ofattenuation to minimizing surface defects such as in U.S. Pat. Nos.3,440,030 (Thompson et al.); 3,533,772 (Itakura et al.); and 3,520,672(Greenler et al.), but it is now believed that none of these approachesfully meets the problem.

SUMMARY OF THE INVENTION

In the aforementioned copending application Ser. No. 137,329, it isdisclosed that applying attenuating forces to a ribbon of glass in afloat chamber in a specific sequence can substantially reduce the amountof apparent optical distortion in below equilibrium thickness glass. Inone embodiment, the sequence may be summarized as passing the glassfirst through a relaxation zone, then a longitudinal and lateralstretching zone before the glass has cooled sufficiently to becomedimensionally stable. Another embodiment is characterized by passing aglass ribbon through a longitudinal stretching zone and subsequentlythrough a lateral stretching zone. In the longitudinal stretching zone,the glass is attenuated in the longitudinal direction as it ismechanically restrained from shrinking substantially in width so thatthe attenuation occurs largely by virtue of thickness reduction. Asubstantial portion, e.g., about 50 percent, of the overall thicknessreduction is effected in the longitudinal stretching zone. It isbelieved that most of the surface defects are imparted to the glassduring this longitudinal stretching. Immediately following thelongitudinal stretching zone is a lateral stretching zone in whichmechanical forces are applied to the ribbon to increase its width whilereducing the ribbon to its final thickness. Stretching in the lateralstretching zone is primarily in the lateral direction, but tractiveforce applied to convey the ribbon in the longitudinal direction isusually sufficient to at least prevent longitudinal shrinking. Followingthe lateral stretching zone is a quiescent zone in which the glassribbon is permitted to cool to a condition at which it may be withdrawnfrom the float chamber without damaging its surfaces.

This particular sequence of attenuating a glass ribbon is designed tominimize the creation of observed surface distortion in the glassproduced. The improvements are based on the recognition that the opticalpower of glass surface distortion is a strong factor of the spatialfrequency of the distortion features in accordance with the followingrelationship which relates optical power of a defect to its geometry:

    P=khf.sup.2

where P is optical power, k is a constant, h is the height or amplitudeof the surface defect, and f is the spatial frequency of the distortionpattern. It can be seen from this relationship that while amplitude andfrequency both affect the optical power, the frequency is a second powerfactor whereas the amplitude is merely a linear factor. Therefore, theprimary objective is to reduce the frequency of the distortion features.However, additional improvement due to amplitude reduction is alsoattained.

The improved attenuation technique is also based on the finding thatlongitudinal stretching is not only a major source of mechanicalperturbations, but even more significantly, serves to increase thefrequency of surface defects introduced into or pre-existing in theglass ribbon. Accordingly, the longitudinal stretching zone of thepresent method preferably is preceded by the relaxation zone so as tominimize the effects of any perturbations on the glass entering thelongitudinal stretch zone. Furtheremore, by carrying out most of thelongitudinal stretching, which is accompanied by some of the mostharmful perturbations, at a point where the ribbon is relatively narrowin width permits subsequent operations to be performed on the ribbonwhich reduce the amplitude and frequency of the surface distortionproduced by the longitudinal stretching. More specifically, by wideningthe ribbon in the subsequent lateral stretching zone, the distortionpatterns produced by the longitudinal stretching also become stretchedin the lateral direction, thereby reducing their frequencies as well astheir amplitudes.

The present invention is an improvement on the method of the previousapplication wherein preferred temperature ranges for the longitudinalstretching and lateral stretching steps have been determined.Longitudinal stretching is carried out at glass temperatures below 1700°F. (925° C.), preferably from 1550° F. to 1650° F. (840° C. to 900° C.).Lateral stretching is carried out in the range of 1450° F. to 1600° F.(790° C. to 870° C.), preferably 1450° F. to 1550° F. (790° C. to 840°C.). Qualitatively, this represents carrying out the lateral stretchingstep at relatively low temperatures, which is advantageous forminimizing the "differential stretching" effect. In other words, lateralstretching at relatively low temperature results in attenuation of theglass ribbon more by thickness reduction and less by reduction of thelongitudinal dimension. Barriers within the molten metal may be used toaid in establishing the desired distinct temperature zones.

A variety of commercial configurations for forming flat glass on bathsof molten metal may be adapted to practice the present invention, andseveral embodiments of such adaptations will be described herein.

THE DRAWINGS

FIG. 1 is a schematic cross-sectional side view of a preferredembodiment of the present invention employed in conjunction with afreefall molten glass delivery system.

FIG. 2 is a schematic plan view of the glass forming chamber of FIG. 1.

FIG. 3 is a schematic cross-sectional side view of an alternate glassforming chamber embodiment of the non-freefall type incorporating thefeatures of the present invention.

FIG. 4 is a schematic plan view of the glass forming chamber of FIG. 3.

FIG. 5 is an enlarged plan view of edge cooling means that may beemployed with the present invention.

FIG. 6 is a side view of the edge cooling means of FIG. 5.

DESCRIPTION OF THE PREFERRED EMBODIMENT

The embodiments depicted in FIGS. 1 and 2 relate to the type of floatglass forming embodiments disclosed in U.S. Pat. Nos. 3,083,551 and3,220,816 (Pilkington) which are in wide commercial use. Details of itsconstruction and operation will be familiar to those of skill in theart. Generally, a mass of molten glass 10 from a melting furnace (notshown) is delivered by way of a canal 11 to a forming chamber 20. Atweel 14 extending through the roof 12 of the canal control the rate ofdelivery of the molten glass to the forming chamber. The chamber maycomprise refractory floor 21, roof 22, and walls 23. A pool of moltenmetal 25 consists essentially of tin or an alloy thereof. The moltenglass enters the forming chamber over a lip member 15 where it fallsfreely onto the molten metal to form a meniscus 26 which is permitted tospread laterally to the extent permitted by surface tension forces ofthe molten glass. The glass need not fall freely from the lip 15 but maybe supported between the lip and the molten metal surface by arefractory member such as that shown in U.S. Pat. No. 4,055,407(Heithoff et al.). This laterally spreading portion of the molten glassis designated zone A in FIG. 2 and constitutes the relaxation zone ofthe present invention. In zone A the glass is either at or aboveequilibrium thickness and is maintained at or above about 1600° F. (870°C.) up to a typical delivery temperature of about 2000° F. (1090° C.).

The principal function of zone A in the present invention is to maintaina relatively long residence time for the glass at this relatively hightemperature range at which the glass will have a relatively lowviscosity, which in turn encourages equilibrium of flow perturbationsarising from delivery of the molten glass onto the pool of molten metal.This relatively long residence time is achieved by providing arelatively large volume of molten glass in zone A, such as by permittingthe glass to spread laterally as shown in FIG. 2. Alternatively, theincreased volume may be attained by enhancing the depth of the glass inzone A by means of side barriers or other means to urge the glassinwardly.

In FIG. 2, zone B represents a longitudinal stretching zone. The glassribbon is at approximately the equilibrium thickness at the point whereit is drawn into zone B. The longitudinal stretching of zone B isinitiated at a location where the glass temperature is below 1700° F.(925° C.). Preferably, the longitudinal stretching is carried out at1550° F. to 1650° F. (840° C. to 900° C.), optimally at about 1600° F.(870° C.). The temperature of the glass ribbon is permitted to fall asit passes through zone B but the temperature is controlled so that thetemperature is not below 1500° F. (815° C.) when it enters thesubsequent zone, zone C. Glass is drawn from zone A into zone B in thelongitudinal direction whereby forces are applied to the glass whichtend to cause the glass ribbon to be reduced in width and thickness.However, the reduction of width would be much more pronounced than thereduction in thickness if the longitudinal attenuation were permitted toproceed without restriction. This is disadvantageous since the ultimateobject of attenuation is to reduce the glass thickness, and since anarrow glass ribbon is less useful commercially. Therefore, means areprovided in zone B to restrict the narrowing of the ribbon and to forcethe attenuation in zone B to take place primarily at the expense of thethickness of the ribbon. The width-controlling means is preferably a setof rotating rolls 28 as shown in the art such as gas jets, blades orelectromagnetic means. Preferably, the rolls 28 may be of the particulardesign shown in U.S. Pat. No. 3,929,444 (May et al.). A plurality ofsets of rolls are provided in zone B so as to maintain the width of theribbon substantially constant, each set consisting of a pair of rolls onopposite sides of the ribbon. The rolls engage the top surface of theedges of the ribbon, and their speeds of rotation are controlled so asto accelerate the longitudinal velocity of the ribbon as it passesthrough zone B. It is preferred that the rolls in zone B be angledoutwardly slightly (about 5° to 10° from the direction of glass travel).In zone B the thickness of the glass is reduced from approximately theequilibrium thickness to a substantially reduced thickness typically onthe order of about halfway or more toward the desired final thickness.This longitudinal attenuation is believed to induce a substantial amountof surface distortion in the glass, but that this distortion constitutesthe majority of the distortion produced by the overall attenuationprocess.

Subsequently, the glass enters a lateral stretching zone, designated aszone C in FIG. 2, where the glass is brought to its final thickness. Theglass in zone C may range in temperature from about 1600° F. (870° C.)to about 1450° F. (790° C.). Preferably, lateral stretching is carriedout at glass temperature between 1450° F. (790° C.) and 1550° F. (840°C.), optimally at about 1500° F. In this final attenuation step thethickness reduction is achieved primarily by increasing the width of theribbon. Lateral stretching forces are provided by means engaging theedges of the ribbon, such as sets of rolls 29 which may be the samedesign as rolls 28, or other known attenuating devices. The rolls 29 areangled so as to impart a lateral component of force to the glass ribbon.Longitudinal force is also applied to the glass in zone C by means ofthe rolls 29 as well as by the conveying means acting upon the formedribbon beyond the exit of the forming chamber. The application oflongitudinal force in zone C is desirable to assure that the finalattenuation is accomplished through thickness reduction rather than byshortening of the longitudinal dimension. Some acceleration in thelongitudinal direction may be imparted to the ribbon in zone C so as tostretch the ribbon in both the longitudinal and lateral directions, butthe longitudinal stretching in zone C should be minor relative to thatimparted to the glass in zone B.

The ratio of the final ribbon width to the ribbon width in zone B isdirectly proportional to the frequency of the optical power of thedistortion. Therefore, it is desirable to maximize lateral attenuationin zone C. It has been found that a dominant distortion pattern due tothickness variation having a frequency ranging from about 0.70 to about0.80 cycles per inch (0.28 to 0.32 cycles per centimeter) is created bylongitudinal attenuation as in zone B. This frequency of opticaldistortion unfortunately happens to be in a region of frequencies whichare highly sensitive to the human eye. The lateral attenuation in zone Cadvantageously reduced this frequency in accordance with the followingrelationship:

    f.sub.2 =f.sub.1 ×W.sub.B /W.sub.D

where f₁ is the optical distortion frequency entering zone C, f₂ is theoptical distortion frequency of the final glass product, W_(B) is thewidth of the glass ribbon in zone B, and W_(D) is the width of the glassribbon in zone D. Accordingly, it is desirable to increase the ribbonwidth in the lateral attenuation zone C to at least 1.05 times the widthof the ribbon in zone B, preferably by a factor of 1.1, and mostpreferably by a factor of 1.5 or higher. When feasible, it is desirablefor the final ribbon width to exceed the maximum width of the glass inthe relaxation zone.

After lateral attenuation, the glass ribbon enters zone D in FIG. 2where it is permitted to cool without further attenuation to atemperature, typically about 1100° F. (595° C.), at which it isdimensionally stable and sufficiently hardened to be lifted from thepool of molten metal by means of lift-out rolls 31 at the exit lip 30 ofthe float chamber. Subsequently, the glass ribbon is typically conveyedon a roller conveyor through an annealing lehr.

In order to assist in establishing the thermal conditions disclosedabove for zones B and C, it is preferred to employ barriers 35 and 36submerged in the tin at the approximate boundaries between zones B and Cand between zones C and D as shown in FIG. 2. The barriers may be of aknown construction, such as the movable barriers disclosed in U.S. Pat.Nos. 3,930,829 (Sensi) and 4,099,952 (Schwenninger), and may be atwo-piece construction as shown to facilitate insertion into the tinbath. The height of the barriers is less than the depth of the tin toavoid contacting the glass, but sufficient to retard the flow of tin inthe longitudinal direction. By retarding the flow of tin from one zoneto another, the desired thermal conditions can be established morereadily in the respective zones. In order to help estabish the desiredthermal conditions in a portion of the glass ribbon, the temperature ofthe adjacent molten metal is usually maintained slightly lower,typically about 30° F. (17° C.) lower.

FIGS. 3 and 4 depict an adaptation of the present invention to a"constant width" type forming process as disclosed in U.S. Pat. No.3,843,346 (Edge et al.). This embodiment differs from the embodiment ofFIGS. 1 and 2 in that molten glass is delivered onto the molten metal inthe forming chamber by means of a wide threshold and without free fallor unhindered lateral spread. Molten glass 40 is contained in a meltingfurnace 41 provided with a metering tweel 43 at the junction between themelting furnace and the forming chamber 50. A wide threshold 44underlies the metering tweel 43 and supports the glass during itsdelivery into the forming chamber until it is supported by the moltenmetal 55. The forming chamber 50 may consist of a bottom 51, roof 52,and sidewalls 53 of conventional construction in the art. In accordancewith the present invention, the glass ribbon 57 passes in sequencethrough four zones designated Q, R, S, and T, in FIG. 4 and whichrespectively correspond in function to zones A, B, C, and D describedabove in connection with FIG. 2. Zone Q is the relaxation zone where, aspreviously described, the glass is maintained relatively undisturbed ata relatively high temperature in order to reduce the volumetricnonuniformities in the newly delivered layer of molten glass. Lateralspread in zone Q is restricted by means of side barriers 56 and 57.

Referring again to FIG. 4, the glass, after leaving relaxation zone Q,enters longitudinal stretching zone R wherein the ribbon is subjected tolongitudinal attenuation to substantially reduce its thickness whilemaintaining substantially constant width by means of edge roll members58 in the same manner described above in connection with zone B in FIG.2. Likewise, subsequent lateral attenuation in a lateral stretching zoneS, which includes outwardly angled edge roll means 59, is carried out inthe same manner as in zone C described in connection with the FIG. 2embodiment above. The temperature of the glass in zones Q, R and S isthe same as that described above for zones A, B, and C respectively.Finally, the glass is permitted to cool, typically to a temperature ofabout 1100° F. (595° C.), in a cooling zone T after which thedimensionally stable ribbon of glass is lifted over exit lip 60 by meansof liftout rolls 61.

A variation of the invention entails passing the glass from a relaxationzone such as A, or Q, as in the previously described embodiments into acombined longitudinal and lateral attenuation zone. In such a zone, thelateral and longitudinal attenuation may be carried out substantiallysimultaneously so that the ribbon of glass is increased in width toessentially its final width and is decreased in thickness to essentiallyits final thickness during passage therethrough. In such an embodiment,a substantial amount of longitudinal stretching would not be performedsubsequent to the final lateral stretching.

Edge coolers 39 are shown in the lateral stretching zones of theembodiments of FIGS. 1 and 2 and FIGS. 3 and 4. The edge coolers serveto increase the viscosity of the edge portions of the glass ribbon,thereby permitting greater traction by the edge stretching rolls 29 or59 on the glass ribbon, which is advantageous for imparting lateralstretching forces on the glass ribbon. Normally, edge stretching rollspull a portion of the glass ribbon outwardly, and between the rolls theribbon shrinks back partially to its original width, thereby producing ascalloped edge effect in a stretching zone. Cooling the edge portions ofthe glass ribbon reduces this scalloping, and as a result more uniformforces are applied to the ribbon. Another advantage of cooling the edgesin the transverse stretching zone is that the relatively stiff edgestransmit to the longitudinal stretching zone more of the longitudinalforces produced by the conveying means downstream from the exit of theforming chamber. Thus, the longitudinal tractive forces may serve tosaid stretching in the longitudinal zone with a diminished effect on theribbon in the transverse stretching zone. Preferably, the marginal edgeportions of the ribbon are cooled to about 50° F. (28° C.) below thetemperature of the central portion of the glass ribbon. Thus the edgeportions typically may range from about 1400° F. to 1500° F. (760° C. to820° C.) after being cooled in the transverse stretching zone.

Suitable edge cooling means are known in the art, an example of which isdisclosed in U.S. Pat. No. 3,692,508. A preferred arrangement comprisesa plurality of water-cooled members 39 shown in FIGS. 1-4, details ofwhich are illustrated in FIGS. 5 and 6. Each cooler 39 includes ahairpin bent water conduit 65, to the end of which is welded a metalplate 66. Beneath the plate 66 is affixed a graphite block 67 whichserves to prevent sticking of the glass ribbon 27 to the cooler in theevent of accidental contact. The conduit 65 may be mounted in a sideseal unit 68 which may be inserted into the customary access openings inthe side walls 23.

The use of edge coolers in combination with a bidirectional attenuationprocess is the subject of a copending U.S. Patent Application Ser. No.307,815 entitled "Method of Bidirectionally Attenuating Glass in a FloatProcess with Edge Cooling " filed by J. R. Mouly on Oct. 2, 1981.

Other variations and modifications employing features known in the artwill be apparent to those of skill in the art and are within the scopeand spirit of the invention as defined by the following claims.

I claim:
 1. A method of making a continuous sheet of glass of belowequilibrium thickness comprising the steps of:delivering a stream ofmolten glass at a first end of a longitudinally extending pool of moltenmetal onto a first zone of the molten metal pool at a rate such that theglass thickness in the first zone is maintained greater than equilibriumthickness; drawing a ribbon of glass in a horizontal longitudinaldirection from the first zone onto a second zone of the molten metalpool and stretching the ribbon in the second zone in a longitudinaldirection while being restrained against shrinkage in the transversedirection while the glass is at a temperature below 1700 ° F. (925° C.)so that substantial reduction of the glass thickness to a thicknessbelow equilibrium is effected in the second zone, and deviations fromsurface flatness of the ribbon are drawn into elongated distortionsextending substantially longitudinally; further drawing the ribbon ofglass in the longitudinal direction from the second zone onto a thirdzone of the molten metal pool and stretching the ribbon in the thirdzone in the transverse direction while the glass is at a temperature of1450° to 1600° F. (790° to 870° C.) so that additional reduction ofglass thickness is effected, and the longitudinally extending surfacedistortions are stretched in the transverse directions, thereby reducingtheir spatial frequencies; further drawing the ribbon of glass in thelongitudinal direction from the third zone onto a fourth zone andcooling the glass in the fourth zone sufficiently to be dimensionallystable; and withdrawing the dimensionally glass ribbon of less thanequilibrium thickness from a second end of the pool of molten metal,whereby a ribbon of glass is produced in which light transmitted throughthe glass is affected by optical distortion of reduced optical power. 2.The method of claim 1 wherein the temperature of the glass is maintainedat 1600° F. (870° C.) to 2000° F. (1090° C.) in the first zone; at 1550°F. (840° C.) to 1650° F. (900° C.) in the second zone; and at 1450° F.(790° C.) to 1550° F. (840° C.) in the third zone.
 3. The method ofclaim 1 or 2 wherein the glass is maintained in the first zone for asufficient residence time to substantially damp volumetric flowfluctuations accompanying delivery of the glass onto the molten metal.4. The method of claim 1 or 2 wherein the glass ribbon is additionallystretched in the longitudinal direction in the third zone.
 5. The methodof claim 1 or 2 wherein the glass ribbon is stretched laterally in thethird zone to a width at least 1.05 times the width of the glass ribbonin the second zone.
 6. The method of claims 1 or 2 wherein the glassribbon is stretched laterally in the third zone to a width at least 1.1times the width of the glass ribbon in the second zone.
 7. The method ofclaim 6 wherein the glass ribbon is stretched laterally in the thirdzone to a width greater than the width of the glass in the first zone.8. A method of making a continuous sheet of glass of below equilibriumthickness comprising the steps of:delivering a fluid stream of moltenglass at a first end of a longitudinally extending pool of molten metalonto an initial zone of the molten metal pool at a rate sufficient tomaintain a mass of relatively fluid glass at a thickness greater thanequilibrium thickness in the initial zone, and providing sufficientresidence time in the initial zone to substantially damp volumetricglass flow fluctuations which may accompany delivery of the glass ontothe molten metal; drawing a ribbon of the glass in a horizontallongitudinal direction from the mass of glass in the initial zone to anattenuation zone of the molten metal pool, permitting the glasstemperature to fall within the range of 1450° F. (790° C.) to 1700° F.(925° C.) in the attenuation zone, and stretching the ribbon in theattenuation zone in the longitudinal direction and simultaneously orsubsequently in the transverse direction to a sufficient extent toincrease the width of the ribbon and to effect a substantial reductionof the glass thickness below equilibrium in the attenuation zone, sothat as elongated surface distortions of the ribbon are induced by thelongitudinal stretching, their spatial frequencies are reduced by thetransverse stretching; further drawing the ribbon of glass in thelongitudinal direction from the attenuation zone into a cooling zone andthere cooling the glass sufficiently to be dimensionally stable; andwithdrawing the dimensionally stable glass ribbon of less thanequilibrium thickness from a second end of the pool of molten metal,whereby a ribbon of glass is produced in which light transmitted throughthe glass is affected by optical distortion of reduced severity.
 9. Themethod of claim 8 wherein the temperature of the glass is maintained at1600° F. (870° C.) to 2000° F. (1090° C.) in the initial zone and at1450° F. (790° C.) to 1650° F. (900° C.) in the attenuation zone.